Cosmic rays

1. Cosmic rays ASTR 3010 Lecture 23

2. History of Cosmic Rays: 1785-1902 • 1785 Charles Coulomb • Discovered that charged body in the air becomes discharged“there are ions in the atmosphere” • 1902 Rutherford, McLennan, Burton • Discovered that penetrating radiation passes through the atmosphere

3. Discovery of Cosmic Rays From 1911 to 1913 Victor Hess measured radiation levels at various altitudes (up to 17,500 ft) in the Earth’s atmosphere. Radiation levels increased with altitude! This radiation was called “Cosmic Radiation” later became “Cosmic Rays”. Won Nobel Prize in 1936. http://hires.physics.utah.edu/reading/intro.html

4. History of Cosmic Rays: 1933-1937 • 1933 Sir Arthur Compton • Radiation intensity depends on magnetic latitude • 1937 Street and Stevenson • Discovery of the muon particle in cosmic rays (207 x heavier than an electron) http://www.sciencebulletins.amnh.org search: Earth’s magnetic shield

5. History of Cosmic Rays: 1938 • Pierre Auger and Roland Maze • Rays in detectors separated by 20m (later 200m) arrive simultaneously • This is known as coincidence

6. History of Cosmic Rays: 1982 • Sekido and Elliot • Gave the first correct explanation of what Cosmic Rays are: ionized atoms (nuclei) from space hitting the atmosphere

7. What are cosmic rays (CRs)? • As it turns out, these charged particles are atomic nuclei zooming through space • Called “primary” CRs • Mostly protons or a (He) nuclei (other elements too, in much shorter supply) • There are more coming in at lower than higher energies • When these hit another nucleus in the atmosphere and stop, more particles are knocked downward, causing a cascading effect called a “shower” • Particles in the shower are called “secondary” CRs

8. Cosmic Ray “Showers” Space “Primary” Cosmic Ray (Ion, for example a proton) Earth’s atmosphere Atmospheric Nucleus p+ p- po “Secondary” Cosmic Rays... (about 50 produced after first collision) po g g p+ p- e+ e- g g m+ muon nm neutrino g e- Creating: Electromagnetic Shower Hadronic Shower Plus some:Neutrons Carbon-14 (mainly muons and neutrinos reach earth’s surface) (mainly g-rays)

9. Primary Cosmic Rays • Composition • Mostly proton (~85%) • 10 Helium nuclei • 1% heavier • 1% electrons and positrons • Lower-energy CRs are common, while high-energy CRs are rare Common enough to be observable by satellite LHC can accelerate a proton to 7 TeV Energies achieved by Man made accelerators

10. Li, Be or B Cosmic ray (proton or α) C, N, or O (He in early universe) What the elements are in the universe • There are more cosmic rays of certain elements than there should be • Due to collisions with other atoms somewhere in space! • These collisions are a major source of lithium, beryllium and boron in the universe

11. CR interactions • Spallation • ISM ionization • Bremsstralhung : mostly electrons passing other ions • Synchrotron loss • Inverse Compton loss : relativistic electrons vs radiation field • GZK effect : Greisen-Zatsepin-Kuzmin effect, for highly relativistic particles, CMB photons appear to be gamma-ray photon  mean free path~10Mpc

12. How do we detect cosmic rays? • To detect secondary showers, observatories are put on the ground • Good: they are cheaper, bigger, and detect a lot more! • Bad: it takes some work to figure out what the primary is like. But it can be done to some extent! • Can either detect the particles, or look for the light as those particles bounce off the air and create fluorescence • To detect primaries, observatories are put in space • Good: it studies the original cosmic ray w/o interference from the atmosphere • Bad: it is an expensive detector that is too small to “catch” a lot of CRs

13. When it comes to CR detectors… BIGGER = BETTER • They catch more cosmic rays overall • Detect more of the ones that are rare! Ultra-high-energy cosmic rays (UHECRs) with more than 1018eV are found only one per square km per century! • Big area can detect larger shower  from higher-energy CRs • The West Desert provides an ideal location for fluorescence observations. • An altitude of ~4,500 feet where the nearest population centers are more than 30 miles away • Light pollution is mostly blocked by the surrounding mountains. • For 347 days per year, the visibility is better than 10 miles.

14. Particle detector arrays Casa Mia, Utah (pictured below): 1089 detectors spaced 15 meters apart

15. Large observatories STACEE: Albuquerque, New Mexico STACEE uses some of the facility's 212 heliostats to collect Cherenkov light. Cherenkov light is like a sonic boom, but for light. It’s produced by electrons in air showers generated by high energy gamma rays.

16. Where do cosmic rays come from? • Stars produce low-energyCRs • e.g., “Solar wind” ejects protons, a and other particles • Supernovae produce medium-energyCRs • But what could possibly make cosmic rays with E > 1018eV (UHECRs)?

17. Where do cosmic rays come from? Problem:Sources of cosmic rays with E < 1018eVcannot be determined because of their deflection in the galactic magnetic field. Solution (?): ButUHECRs (with E > 1018 eV) are much less deflected (travel straighter) and their direction should point towards their origin Galactic magnetic field M83 spiral galaxy

18. Supernovas: a source of UHECRs? X-ray image by Chandra of Supernova 1006 Blue: X-rays from high energy particles Shockwave from the supernova hits gas surrounding the explosion, possibly accelerating CRs to 1015eV. Not enough energy for UHECRs! Red: X-rays from heated gas (reverse shock)

19. Gamma Ray Bursts Intense Gamma Ray bursts detected by orbiting space telescopes. Burst only last a few seconds. Occur randomly over the entire sky

20. Ultra High Energy Cosmic Rays: The Mystery • The UHECRs we have detected appear to come from all directions, so they probably come from far away (only sources nearby are galaxies in few directions) • Problem: UHECRs should lose energy when they travel • Cosmic microwave background radiation should slow them down or destroy them • GZK cutoff – an upper limit on the energy from distant sources • …butwe’ve detected UHECRs that have much more energy than they should have after coming from far away! • Another problem: we don’t know of any object in the cosmos that would accelerate particles to such high energies (means: no working models)

21. Potential sources of UHECRs? • Colliding galaxies • Super-magnetized spinning neutron stars • Giant black holes spinning rapidly • Gamma ray bursts • Something we haven’t seen yet?

22. The Future of Cosmic Ray Research • We will continue trying to explain how and where UHECRs are produced • We will build bigger detectors on the ground and launch more into space A composite image showing cosmic ray distribution in the sky